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Classification of cathode materials for lithium-ion batteries

by:dcfpower     2021-03-11
u003cpu003eThe cathode material of lithium-ion batteries is called the core of lithium-ion batteries and has always been the focus of research by scientists. In the process of battery charging and discharging, the positive electrode material not only serves as a lithium source, but also provides the lithium needed for reciprocating insertion and removal between the positive and negative lithium insertion materials inside the battery. It also bears the burden of forming the surface of the battery’s negative electrode material. Lithium consumed by the solid-liquid interface membrane (SEI membrane). Therefore, an ideal cathode material needs to have the following characteristics: high potential, high specific capacity, high density (including compaction density and tap density), good safety, good rate performance and long life. u003c/pu003eu003cpu003eu003cpu003eu003c/pu003eu003cpu003eAt present, the materials that can meet the above requirements are mainly divided into three types according to their structural characteristics, namely the layered structure material LiMO2 (Mu003dCo, Ni, Mn) ; Lithium manganese oxide material with spinel structure (LiMn2O4); LiMPO4 with olivine structure (Mu003dFe, Mn, Co, Ni). In recent years, some materials with new structures have also received more and more attention, such as silicate, borate and olivine structure derivatives—Tavorite compounds. 1. Layer structure cathode material At present, LiCoO2 has always been the mainstay in the commercial lithium ion battery cathode material. LiCoO2 has a two-dimensional layered structure of α-NaFeO2 type, which is very suitable for the insertion and removal of lithium ions. It has the advantages of high voltage, stable discharge, high specific energy, good cycle performance, and simple preparation process. It can adapt to high current charge and discharge. Its theoretical capacity is 274 mAh/g. In order to maintain good cycle stability, the actual capacity is controlled to 140 mAh/g. However, LiCoO2 material as a positive electrode has problems such as large battery capacity degradation, poor overcharge resistance, and poor thermal stability. In order to overcome these shortcomings of LiCoO2 materials, methods such as doping modification and coating are often used to improve its stability. The theoretical capacity of layered LiMnO2 is relatively high, 285mAh/g, which has the advantages of high energy density, non-toxicity and low cost. However, in the process of charging and discharging, due to the Jahn-Teller effect, its structure will change, resulting in pulverization of the material and rapid decline of reversible capacity. In order to prepare LiMnO2 with a stable layered structure, other transition metal elements can be introduced on the Mn-O layer to form a composite metal oxide with Mn to enhance the stability of the layered structure of the material. It is reported in the literature that adding Al, Cr, Co, Ni and other elements that can stabilize the layered structure of the material into layered LiMnO2 can significantly improve its electrochemical performanceu003c/pu003eu003cpu003eu003cpu003e. 2. Spinel structure cathode material Spinel LiMn2O4 has the advantages of good overcharge resistance, high thermal stability, rich resources, and environmental friendliness. It is considered to be the most promising cathode material for lithium-ion batteries. However, it has the defect of poor high-temperature cycling performance. Therefore, the research on the modification of spinel LiMn2O4 has always been a hot spot in this type of material. Using Mn3O4 as the synthesis precursor and reacting at 800℃, pure phase spinel LiMn2O4 microspheres with superior electrochemical performance can be obtained; research has found that compared with pure LiMnu003c/pu003eu003cpu003e2O4, the surface is coated LiMn2O4 with YPO4 shows better cycle performance. This is because YPO4 isolates the positive electrode active material from direct contact with the electrolyte, prevents the dissolution of Mn3+, and inhibits the growth of battery impedance, thereby further improving the thermal stability of the electrode . 3. Olivine structure cathode material LiFePO4 has the advantages of good cycle stability, high safety, and green friendliness, and has always been a research hotspot in the field of power lithium-ion batteries. LiFePO4 has a regular olivine structure, belongs to the orthorhombic crystal system, Pmnb space group, the unit cell parameters are au003d0.469nm, bu003d1.033nm, cu003d0.601nm. At present, various methods such as solid phase method, coprecipitation method, sol-gel method, hydrothermal/solvothermal method, microwave method and carbothermic reduction method can be used to synthesize LiFePO4. Due to the low electronic conductivity and ionic conductivity of LiFePO4, the overall electrochemical performance of the material is poor, which is severely limited in practical applications. It can be improved by coating, doping or nanomaterials. Fan et al. prepared LiFePO4/C (LFPC) and LiFe1-2xTixPO4/C (LFTPC) by carbothermal reduction method. The conductivity of carbon-coated and Ti-doped LFTPC can reach ~10-4S/cm; with different ratios of TiO2 Doped with LiFePO4 (LFP), the crystal structure of LFTPC is very stable and has a smaller particle size compared with LFP; among LFTPCs with different Ti doping rates, LFTPC with a doping rate of 2% has the best rate performance And cycle performance. u003c/pu003eu003cpu003eu003cpu003eFigure 1 The theoretical and actual energy density of some lithium battery pack cathode materialsu003c/pu003eu003cpu003eu003c/pu003eu003c/pu003eu003cpu003e Figure 1 shows the lithium-ion battery cathode The theoretical energy density of the material and the energy density in current research. It can be seen that compared with layered structure and spinel structured materials, olivine structured materials and their derivatives have lower energy density. Therefore, for high-energy density lithium ion battery cathode materials, the current research hotspot It is inclined to research on layered structure and spinel structure materials; however, because of its outstanding advantages of high safety, olivine structure materials have always been one of the important research directions of power batteries. u003c/pu003eu003c/pu003eu003c/pu003eu003c/pu003e
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